Investigation of the fibre-matrix interface for glass fibre & carbon fibre composites exposed to cryogenic loading and fatigue conditions.

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Multiple sectors are currently evaluating the use of liquid hydrogen (LH2) as a key future energy source, with particular focus from the aerospace sector. Given the market impetus for wide adoption of LH2 there is significant need to establish rational design rules for the pressurised vessels that will be used to store LH2 safely on the airframe over its lifetime. These vessels, manufactured using composite materials, will be exposed to long-term adverse environmental conditions that have not yet been fully investigated. The effect of cryogenic cycling on the fundamental fibre-matrix performance of a composite material is an unaddressed and critical parameter in the long-term structural performance of LH2 pressurised vessels. Composite properties result from a combination of the characteristics of the fibre and the matrix as well as the ability to transfer stresses across the fibre-matrix interface, which acts as the link between the two components. The strength of the fibre-matrix interface determines how much of the applied stress can be transferred to the load-bearing fibres. This interfacial strength is largely determined by the degree to which the fibres and matrix are intimately contacted and the level of adhesion at those contact points. With the fibre-matrix interface playing a key role in defining the long-term performance of composites deployed in harsh environments, there is now an important need for fundamental studies of the effects that repeated exposure of a composite system to cryogenic conditions will have on the fibre-matrix interface. This in turn will inform us on any changes to the structural and sealing performance of a composite pressurised vessel used to store LH2. As we develop away from the last generation of high-performance thermoset polymers derived from fossil fuels, (e.g. epoxies), towards new sustainable forms of resin system, (e.g. bio-derived resins, liquid thermoplastic resins and/or vitrimers), there is a critical need to understand and optimise the interface interaction for these systems, whilst exposing them to the harsh environments that they will be expected to operate in long-term.The aim of this experimental PhD project is to understand and quantify the fibre-matrix interface for current generation resin (e.g. epoxy) and fibre combinations, (e.g. glass/carbon), utilised in LH2 pressurised vessels. Research would span investigating the effects on the interface whilst at cryogenic temperatures, as well as the effects of repeated cryogenic cycling on the fibre-matrix interface. Comparable work is already conducted within the Advanced Composites Group (ACG). The investigation would encompass micro- (single fibre) and macro-mechanical testing in combination with thermal analysis. The effect of variations in temperature and humidity to simulate operational conditions would also be explored. The scope of the project may also be expanded to include next-generation resin systems, including bio-derived and vitrimer systems, given their future importance in the sector.